Because monitoring tire pressures is essential to the safety of motor vehicles, pressure-monitoring systems, such as TPMS systems, have been created to perform this function and installed on motor vehicles.
These chiefly comprise sensors which measure the pressure in the tires and transfer this measurement to a wheel-monitoring central control unit which is carried on board the vehicle. This central unit collects and analyzes the tire pressure data via wireless links. The result of these analyses is generally displayed on the dashboard of the vehicle. Such a system is therefore able to inform the driver of the tire pressure in real time, or just produce a visual or audio announcement if pressure anomalies are detected by this system.
A stream of various information coming from each wheel unit associated with a wheel is transmitted via radiofrequency link to the wheel-monitoring central control unit of the motor vehicle. The performance of this wireless link is directly impacted by the various vehicle platform profiles, such as, for example, van, sedan, etc. The parameters that need to be taken into consideration may include the length of the vehicle, whether or not the chassis is reinforced, and the materials used in the construction of the vehicle. Certain vehicles will exhibit zones in which transmission is deficient, these being more commonly referred to as “black spots”, in which the wireless signal is not transmitted.
These black spots are highly problematic. Specifically, if a frame of information is transmitted and finds itself in such a black spot, the frame is lost. This is of particular relevance in assistance with the inflation of tires of a vehicle, which is performed when the vehicle is stationary, this being referred to as being in immobilized mode. In these instances in which the vehicle is stopped, if the wheel unit is in a black spot, this spot remains the spot from which the wheel unit transmits throughout the time for which the vehicle is stopped. The level of reception of the signals at the wheel-monitoring central control unit drops to the point at which these signals may become inaudible and prevent the inflation assistance from working.
These black spots may be caused for example by the position of the transmitter antenna of a wheel unit, by reflection off the ground of part or of all of the transmitted signal, or by the relative position of the wheel unit with respect to the wheel-monitoring central control unit.
In order to alleviate this problem of black spots, standard protocols have been devised which send identical frames in packets of two, three or four or even more in order to improve the information reception rate. The probability of the information not being transmitted decreases with the number of frames transmitted. The counterpart to this is increased battery power consumption and therefore a shorter life for the wheel unit.
A protocol has also been devised that has a standard fixed inter-frame time for which the reception quality is dependent on a predetermined sensitivity of the reception of information transmissions between a wheel unit and the wheel-monitoring central control unit. However, there is no correlation between such a reception assessment criterion and the number of frames actually transmitted, which means that this criterion is not objective.